This invention relates to optical circulators, isolators or switches and to systems incorporating such devices. Of particular interest are hybrid verions of such devices contained within an easily-manufactured subassembly.
With the advent of single mode optical fibers and their rapid acceptance into a variety of actual and proposed optical communications systems (e.g., fiber to the home (FITH), and coherent detection systems), the need for nonreciprocal components, such as optical isolators and circulators, has grown. For these components to be practical they should not only be easily and inexpensively manufacturable but also exhibit low insertion loss at the transmission wavelength(s) and high isolation for unwanted signals (e.g., reflections). Isolation is particularly critical in many systems which are sensitive to spurious reflections. For example, distributed feedback (DFB) lasers tend to be unstable if reflections of the laser output are coupled back into the laser cavity. While expense is important in nearly all applications, it is particularly critical in high volume, low cost projects such as FITH. Isolation and expense are also issues in systems using optical amplifiers where reflections can cause an amplifier to oscillate.
In the past bulk optical circulators and isolators have been proposed, but these components are costly to produce with the required low insertion loss for single mode transmission systems. More recently, some workers in the art have attempted to reduce the number of components by combining the lensing and polarization rotation functions into a single component. The following articles are representative:
I. Yokohama et al, Electron Letters, Vol. 22, No. 7, pp. 370-372 (1986) entitled "Polarisation-Independent Optical Circulator Consisting of Two Fiber-Optic Polarising Beam Splitters and Two YIG Spherical Lenses" discloses a single mode circulator in which the lensing and polarization rotation functions are combined in spheres of Faraday rotation material (YIG) and in which a polarization beam splitter/combiner is made of birefringent fiber. Isolation (-18 dB) suffered because the Faraday rotation angle, which is related to optical path length, is a function of whether a light ray passes through the center of the sphere or not. Moreover, insertion loss (2.5 dB) was relatively high and the fiber beam splitters were difficult to fabricate, making the design unsuitable for many of the applications discussed above.
K. Drogemuller, Journal of Lightwave Technology, Vol. 7, No. 2, pp. 340-346 (February, 1989) entitled "Compact Optical Isolator With a Plano-Convex YIG Lens for Laser-to-Fiber Coupling" discloses a single mode isolator (for a DFB laser) in which the lensing and polarization rotation functions are combined in a plano-convex body of Faraday rotation material (YIG). A confocal two-lens system is formed by the plano-convex YIG lens and a spherical glass lens. Although isolation of greater than 47 dB was obtained, the system requires a polarizer and critical alignment of the confocal system both of which would increase insertion loss (not reported) and complexity. Moreover, the use of many relatively large components with different expansion coefficients strongly implies a lack of mechanical stability (and component alignment) in environments (e.g., FITH) where wide temperature variations are experienced.
Thus, there remains a need in the art for single mode optical circulators and isolators which have relatively low insertion loss and high isolation, yet can be relatively easily manufactured and can be expected to exhibit good mechanical stability.